As such a kind of flow rate measuring device, the device to be described below with reference to FIG. 5 is generally known (see patent literature 1, for example). FIG. 5 is a block diagram of a conventional flow rate measuring device.
As shown in FIG. 5, the conventional flow rate measuring device is formed of flow path 21 through which fluid to be measured flows, first ultrasonic transducer 22 and second ultrasonic transducer 23 disposed on flow path 21, switching part 24, transmitter 25, amplifier 26, reference comparator 27, determining part 28, timer 29, and controller 30.
Switching part 24 switches transmission/reception between first ultrasonic transducer 22 and second ultrasonic transducer 23. Transmitter 25 drives first ultrasonic transducer 22 and second ultrasonic transducer 23. Amplifier 26 amplifies a received signal, which is received by the receiver-side transducer (i.e., either first ultrasonic transducer 22 or second ultrasonic transducer 23) and is passed through switching part 24, to predetermined amplitude. Reference comparator 27 compares the voltage of the received signal amplified by amplifier 26 with reference voltage.
Determining part 28 will be described below with reference to FIG. 6. Receiving the result of comparison between received signal A amplified by amplifier 26 and reference voltage from reference comparator 27, determining part 28 detects zero cross point a of received signal A immediately after received signal A exceeds the reference voltage. According to the detection timing by determining part 28, timer 29 measures a propagation time required for transmission/reception of an ultrasonic wave. Controller 30 controls transmitter 25 and amplifier 26; and at the same time, calculates flow velocity v and/or flow rate Q of fluid according to the propagation time measured by timer 29.
Hereinafter, measurement operation of flow rate Q of fluid to be measured with use of a conventional flow rate measuring device will be described with reference to FIG. 5.
First, the flow rate measuring device requests controller 30 to drive transmitter 25 so that transmitter 25 drives first ultrasonic transducer 22 (at the time, first ultrasonic transducer 22 has been switched to the transmitter side by switching part 24). Through the driving operation, an ultrasonic signal transmitted from first ultrasonic transducer 22 travels through the fluid in flow path 21 to second ultrasonic transducer 23. The ultrasonic signal received by second ultrasonic transducer 23 is amplified by amplifier 26 and carried to reference comparator 27 and determining part 28 for signal processing. The ultrasonic signal after signal processing is fed into timer 29.
Next, after switching part 24 switches the transmitter/receiver side between first ultrasonic transducer 22 and second ultrasonic transducer 23, the aforementioned procedures are performed in a similar manner.
Through the procedures above, timer 29 measures each propagation time in the fluid flowing from the upstream to the downstream (as a direction of the forward flow) and in the fluid flowing from the downstream to the upstream (as a direction of the backward flow).
Through the measurement above, flow velocity v of the fluid is obtained and flow rate Q is calculated by expression 1 below:Q=S·v=S·L/2·cos φ(n/t1−n/t2)  (expression 1),
where, L represents an effective distance between first ultrasonic transducer 22 and second ultrasonic transducer 23 in the flowing direction; t1 represents a propagation time in the forward flow; t2 represents a propagation time in the backward flow, v represents a flow velocity of the fluid; S represents a sectional area of flow path 21; φ represents a sensor angle; and Q represents a flow rate. Sensor angle φ is the angle (in FIG. 5) formed between the single-lined arrow (indicating the traveling path of the ultrasonic wave between first ultrasonic transducer 22 and second ultrasonic transducer 23) and the outlined arrow (indicating the flowing direction of the fluid in flow path 21).
Practically, flow rate Q is obtained by further multiplying expression 1 by a coefficient suitable for flow rate Q.
In the process above, controller 30 adjusts the gain (i.e., the amplification degree) of amplifier 26 so that the amplitude of the signal received by the receiver-side ultrasonic transducer—either first ultrasonic transducer 22 or second ultrasonic transducer 23—keeps a constant level. This allows the maximum voltage of the received signal to be within a predetermined voltage range.
Hereinafter, a general method of adjusting amplification degree for an ultrasonic signal will be described with reference to FIG. 7.
FIG. 7 illustrates a generally used method of adjusting amplification degree in a flow rate measuring device.
As shown in FIG. 7, during the measurement of an ultrasonic signal, when the maximum voltage of received signal b (shown by a dotted line) becomes lower than the lower limit of the predetermined voltage range, the gain (amplification degree) is adjusted so that the maximum voltage of the received signal increases and gets into the predetermined voltage range in the next flow rate measurement.
Similarly, when the maximum voltage of received signal c (also shown by a dotted line in FIG. 7) becomes higher than the higher limit of the predetermined voltage range, the gain (amplification degree) is adjusted so that the maximum voltage of the received signal decreases and gets into the predetermined voltage range in the next flow rate measurement.
To be specific, when the maximum voltage of the received signal falls below the lower limit, the amplification degree is increased so that the maximum voltage takes a value between the higher limit and the lower limit of the voltage range, just like received signal a shown by a solid line in FIG. 7. Similarly, when the maximum voltage of the received signal exceeds the higher limit, the amplification degree is decreased so that the maximum voltage takes a value between the higher limit and the lower limit of the voltage range, just like received signal a.
In this way, the amplification degree of a detected ultrasonic signal is adjusted by the method above.
The reference voltage of reference comparator 27, which is to be compared with the received signal amplified by amplifier 26, is used to determine the position of a zero cross point detected by determining part 28.
The reference voltage for determining a zero cross point will be described below with reference to FIG. 6.
FIG. 6 shows an example of operation for determining zero cross point a from a received signal in a conventional flow rate measuring device.
As shown in FIG. 6, for example, the reference voltage is set to the center point of the difference between the positive peak voltage of the third waveform and the positive peak voltage of the fourth waveform of the wave of the received signal that travels in the air flowing in flow path 21. By virtue of the setting above, even when the peak voltage of the third waveform of the received signal increases or when the peak voltage of the fourth waveform decreases due some reason, the reference voltage has a margin with respect to each peak voltage. This enables determining part 28 to have a stable detection of zero cross point a of the fourth waveform.
However, according to a conventional flow rate measuring device, the reference voltage is set to a fixed value. That is, as shown in FIG. 6, to obtain a stable detection of zero cross point a, the reference voltage is set to a fixed value that corresponds to the center point of the peak voltages having the largest interval therebetween of the received signal traveling in the air, i.e., between the third waveform and the fourth waveform of the wave. Therefore, if the fluid to be measured is changed from air to other gases, the waveform of a received signal depends on the types of gas and it can largely change from the waveform of the received signal traveling in the air (FIG. 6).
As a result, if the peak voltage of the third waveform of a received signal increases largely and exceeds the reference voltage, the zero cross point of the third waveform is wrongly detected as zero cross point a. Similarly, if the peak voltage of the fourth waveform of a received signal largely decreases to a value smaller than the reference voltage, the zero cross point of the fifth waveform is wrongly detected as zero cross point a.
That is, according to the conventional flow rate measuring device, the reference voltage is set in advance to the center point between the peak voltage of the third waveform and the peak voltage of the fourth waveform of a received signal traveling in the air. With the structure above, when the fluid to be measured is air or when the fluid to be measured is a gas that has a small change in the waveform of the received signal from the waveform in the case of air, the conventional device offers stable detection of the zero cross point of the fourth waveform and therefore provides flow rate measurement with high accuracy.
However, if the fluid to be measured is a gas that has a big change in the waveform of the received signal from in the case of air, the detection point for detecting a propagation time has variation—as described above, the device may wrongly detect the zero cross point of the third waveform or the fifth waveform—and therefore the measurement of the propagation time has poor accuracy. Further, the problem above causes poor accuracy in calculation value of flow rate of fluid to be measured.